Presently available resists usually have one or more drawbacks. For example, most negative acting resists have limited resolution because they swell upon development or, in the case of positive acting, novolac resin based resists, are formulated from materials which are opaque at the shorter wavelengths needed for high resolution work, or are very insensitive as in the case of positive acting, polymethyl methacrylate based resists.
Some problems and recent developments associated with high resolution lithography are described, for example, by M. J. Bowden (ACS Symposium Series 266, American Chemical Society, Washington, D.C. 1984, p. 39-117).
Processes have been developed to circumvent these problems (multi-level resists), but they are complex and require an increase in the number of steps per device layer. An ideal resist would be a single layer which could be exposed with deep UV light (250 to 300 nm) (DUV), X-rays, or electron beam radiation.
In addition to high resolution, which is a measure of how small a structure can be created in the resist, high contrast (defined for a resist as a measure of the resist's sensitivity to changes in exposure dose) is a desirable feature for high resolution resists. This is true because the image formed by the exposure tool is not perfect but has a contrast (defined for the tool as the sharpness of its image) limited by the laws of optics and the size of the structure to be reproduced. The resist must correct the fuzziness of the projected image to give a structure of proper size with sharp vertical walls. This is particularly important for structures having widths less than 11/2.mu. meter, since the resist film is typically 1 to 11/2.mu. meter thick. If the resist wall is not vertical, any subsequent process step which erodes a portion of the resist surface, such as reactive ion etching, will change the width of the structure which in turn can have a deleterious effect on the circuit being produced. The higher the contrast of a resist the more the resist can correct for low contrast in the projected image, and the smaller will be the features of an image which can be successfully exposed to give a usable structure.
High sensitivity is also an essential property of a high resolution resist. The economics of semiconductor device manufacture require high throughput. This means that the time taken to expose each wafer must be minimized. As a result, the exposure energy per unit area of resist is constrained. Furthermore, some exposure tools have less total energy available in the bands used to achieve high resolution than they have in their conventional, low resolution exposure bands. This further reduces the energy available at the wafer suface.
The use of cationic photoinitiators (such as those disclosed in U.S. Pat. Nos. 3,981,897; 4,450,360; or 4,374,086) to cleave polymer pendant groups so as to change the polymer structure to an extent sufficient to create significantly different solubility characteristics in the irradiated and unirradiated areas is disclosed in U.S. Pat. No. 4,491,268. The polymers described as useful in U.S. Pat. No. 4,491,268 are blocked poly-4-hydroxy styrenes, a blocked poly 4-vinylbenzoate, blocked poly-isopropenylphenyloxyacetates or blocked poly-methacrylates. The blocking groups said to be useful included a wide range of acid labile groups, such as trityl, benzyl or tert-butoxy. This includes structures such as ##STR3## where X and Y are acid labile blocking groups, e.g., ##STR4## Y. The structures of the deblocked polymers present after irradiation and baking are respectively.
In short, there is a need for new resists having high sensitivity, high contrast and high resolution. The need arises from the desire of semiconductor manufacturers to produce structures smaller than 11/4 .mu.m in width.